The Hall A Compton

The Compton polarimeter is used to (non-invasively) monitor polarization of the incoming electron beam.
It has three major components: an optics table with a laser that can lock to a Fabry-Perot cavity, an electron detector, and a photon detector to count scattered electrons and photons, respectively.

Shift Workers' Operation Guide

The Compton DAQ is located in the back room in the Counting House. All commands are done from compton@compton.

If CODA is not already running:

Kill CODA first, then start it up again:

$ kcoda
$ startcoda

A few new Terminal windows and a GUI pop up.

Let "top" refer to the bar with named menu options near the top of the GUI (ex: Platform, Sessions, etc). Let "bottom" refer to the bar below "top" that contains a line of buttons (hover over them to reveal their labels; starting with "Configuration" on the left).

In top, select: Platform > Connect

In bottom, click on "Configuration" button.

If configuration listed in the "Run Parameters" box is not FADC_prod: in top, select Configurations > Cool and change it to FADC_Prod.

In bottom, click on "Download" button

In bottom, click on "Prestart" button

When ready to start run, click "Start" button. Typical run duration: 2 hours. Type in any Start Of Run comments in the box. Click "OK" (even if you left comments blank!).

When ready to stop run, click "End" button. Type in any End Of Run comments in the box. Click "OK" (even if you left comments blank!).

For PREX we should use: /home/compton/online/CompMon/dataQualityCheck.Cas our online macro!

As of now the way to run it is to navigate to the directory with the file and then running with root -l dataQualityCheck.C\(<run number>,<tests flag>\) We're looking into adding a simplified interface in the near future.

The following macros were maintained by people who no longer work on the compton so use with caution:

Helicity controls

On far right of JTabs menu, there are "master" controls. Find and click on Tools Screen.

Go to the helicity controls. (The default is delayed, so be careful of this.)

Current Scripts

As a brief check, run plotAsymSpectrum.C.

PREX-II/CREX Compton Analysis

We've reworked the analysis to streamline finding the most important plots. We've adopted the analysis GUI used by panguin and other online monitors for Hall A experiments. To run, follow the steps below.

Online Analysis Steps and Checks

Using the directions at the top of the page, open CODA and start a run.

Open a terminal and login using ssh -Y compton@compton.jlab.org

Enter command gocompton

First run compmon to analyze the run. To this use command ./compmon.sh -r <run number>

Once the run is finished analyzing and errors are logged, enter source online.csh -r <run number>. This will create the plots in the GUI window.

Look at the plots in the following tabs:

Snapshots: This window has one plot of snapshot peak height vs sum. Plot should look starkly linear. Plot also has the percentage of snapshots that pass an arbitrary selection criteria. I find healthy runs have about 80% of snapshots pass.

Spectra: Plots compton spectrum broken down by beam and laser state. Is the compton edge where we expect it to be? Are Brem events low when beam or laser is off?

Acc0: Plots of accumulator 0. Are they gaussian? Does the mean of the beam ON, laser ON plot differ from all the others?

Asym and Polarization: Includes plots of the quartet sums, differences and asymmetry. Do they all look roughly gaussian? Are there no double-peaking effects? Does the calculated polarization yield some reasonable value?

Asyms vs Time: Plots of the above vs time. Is there any significant drift in any of the quantities vs time?

Asym Spectrum: As of 2019-07-31 this plot is not working. Background subtraction needs to be more carefully refined.

Once you click "Exit GUI" the same plots will be generated and saved to the compton plots web page for easy review.

Making Custom Plots

There are macros which make other compton plots which have been useful in the past, but are not being maintained anymore. To be certain, you can make your own plots to slake your curiosity. The following is description of CompMon output, the branches, as well as certain variables which you may find useful.

General Variables: These variables may be found stored in multiple trees and should come in handy:

helicityStateReported: 0 for negative, 1 for positive

laserState: 0 for right-hand polarization and laser on, 1 for left-hand polarization and laser on, 2 for right hand polarization and laser off, 3 for left hand polarization and laser off, 4 for unknown state

beamState: 0 for beam off, 1 for beam on, 2 for unknown beam state.

cavPowerCalibrated: laser cavity power

bcm: calibrated BCM signal

mpswise: A tree of data stored per MPS

mpsCoda: MPS number

Acc#: Accumulator total for number #

NAcc#: Number of accumulator samples per MPS for accumulator number #

numTriggers: Number of triggered sums in current MPS

triggerwise: A tree of variables stored per trigger

sum: Total sum of trigger

sumIsRandom: 0 if sum is real, 1 if it is random

quartetwise: A tree of variables stored per quartet

PosHelAcc#: Sum of accumulator number # for positive helicity states in the quartet

PosHelNSamples#: Number of samples for accumulator number # for positive helicity states in the quartet

NegHelAcc#: Sum of accumulator number # for negative helicity states in the quartet

NegHelNSamples#: Number of samples for accumulator number # for negative helicity states in the quartet

quartetReportedHelicityPattern: Two different values, one for each valid quartet pattern

snapshots: A list of variables and snapshot readouts for each snapshot taken

numSamples: Number of samples in snapshots

snap: An array variable with numSamples entries that details the fADC readout of the snapshot

pulserwise: A tree of variables stored per pulser cycle. (SHOULD ONLY BE NEEDED BY EXPERTS)

runwise: A tree of variables stored per run. This tree only has one entry per leaf, and details info about pedestals.

epicswise: A tree of variables taken per EPICS event. These variables are also stored in other trees, so they can be accessed from there.

This is in addition to the inbuilt histograms produced in every CompMon output file. This should be more than enough to diagnose any compton DAQ problems.

Expert Information

Analyzer

The Makefile is ancient (since it has to work on old systems too) but there is an easy "fix". In the src/config.d directory, create a file similar to cornejo_compton_jlab.conf but modify the paths respectively.
Then run ./config config.d/<your_config_file>
It will update the silly Makefile accordingly